1. Field of the Invention
The present invention relates to a magnetic recording medium capable of recording both an analog signal and a digital signal.
2. Description of the Related Art
In recent television broadcasting or the like, a digital broadcasting or data broadcasting has been familiar to people. As magnetic recording media for audio and video, goods that meet a digitization have been mainly progressively utilized.
Since an analog record has been currently shifted to a digital record, if both an old analog record and a new digital record can be reproduced and edited by the same VTR or the drive, maneuverability thereof will be improved. Thus, a set of a VTR and a drive which can be used in multiple ways appears on the scene.
As disclosed in below-described Patent documents, in recent digital VTRs for a broadcasting station and a business or drives for data or the like, formats for simultaneously recording time codes as time signals or analog audio signals as well as digital signals are mainly utilized.    [Patent Document 1] Japanese Patent No. 3302397    [Patent Document 2] Japanese Patent Laid-Open Publication No. Heisei 7(1995)-201031    [Patent Document 3] Japanese Patent Laid-Open Publication No. Heisei 7(1995)-192251    [Patent Document 4] Japanese Patent Laid-Open Publication No. Heisei 7(1995)-235044
However, in the digital signals and the analog audio signals, it is difficult to satisfy respective characteristics at the same time due to the difference in their frequency or depth of record and various investigations have been carried out.
For instance, there is a technique that magnetic paint composed of magnetic powder of micro-particles meeting short wavelength suitable for a digital recording is applied to the thickness of 2 to 5 μm as the depth used for an analog recording.
However, this method has a problem of loss of thickness due to a self-demagnetization. For instance, an output is lowered in electromagnetic transfer characteristics because the thickness of a magnetic layer is large.
As a method for solving this problem, such a technique as described below is ordinarily used. That is, a non-magnetic thick layer is provided on the surface of a non-magnetic support member as a lower layer and a magnetic layer is formed on the non-magnetic layer as an upper layer, and the magnetic layer is thin to reduce thickness demagnetization and achieve a high output. However, since the lower layer is non-magnetic, this method is disadvantageous for the analog signal.
As compared with the above-described method, there is an Mag-on-Mag technique that a lower layer is made of a magnetic layer suitable for an analog recording and a magnetic layer suitable for a digital recording is applied thereon to satisfy both the layers. A multi-layered technique of the magnetic layers is characteristically designed to record an analog audio signal whose recording frequency is long as high as 1 KHz is recorded at the position of 2 to 3 μm in the direction of depth of a magnetic recording medium, and, on the other hand, to record a band of several MHz to several ten MHz used for recording a video signal with a shorter wavelength within 0.3 μm of the surface layer of the magnetic recording medium.
That is, according to the above-described Mag-on-Mag technique, the recording frequency of an upper layer has a short wavelength. Therefore, the magnetic powder of micro-particles is used for the upper layer and magnetic powder having such magnetic characteristics as to take an output even in a long wavelength area is used for a lower layer, and the two layers are laminated in two layers.
Various kinds of Mag-on-Mag methods have been hitherto studied and many formats that a video signal is overwritten on an audio signal in a VHS or the like have been studied. However, when the Mag-on-Mag technique is put into practical use, conditions such as sizes of magnetic members combined together in upper and lower layers, magnetic characteristics and the thickness of each layer or the like are strict due to the above-described problem of self-demagnetization.
Here, there is a problem in producing the magnetic recording medium. When two kinds of magnetic paints are applied in layers, fine stripes (streaks) may be formed on the surfaces of magnetic layers owing to the difference in viscosity characteristics, or an upper layer may not be possibly adequately formed on a lower layer or a discontinuous miss of application may be possibly produced due to the difference in wet characteristics between the upper layer and the lower layer, which frequently causes a serious problem. As described above, the combination of the upper layer and the lower layer poor in their compatibility deteriorates a roughness in the surfaces of the magnetic layers to deteriorate an output due to a spacing between a head and the magnetic recording medium. Accordingly, this combination constitutes a problem in practical use.
In order to solve these problems, in a method for simultaneously applying paint on two layers by a Wet-on-Wet technique, the viscosity difference between the upper layer and the lower layer is located within a prescribed range. As an example of this technique, the Patent Document 1 discloses a method for making the viscosity of the upper layer higher by 50 CPS or more than that of the lower layer. According to this method, the generation of stripes is suppressed, however, the generation of a miss of application is not frequently effectively prevented. On the other hand, a Wet-on-Dry system that paint is firstly applied and dried only on a lower layer, and then, paint is applied to an upper layer has been studied. However, either of these methods has not a satisfactory effect.
Further, in the application type magnetic recording medium having a plurality of magnetic layers, paint is easily peeled off depending on the combination of paints of the layers to be laminated. Thus, durability is deteriorated or the output and the C/N of a video is greatly changed due to the roughness of the surface of the magnetic layer of a surface layer. In order to solve these problems, for instance, a method (see Patent Document 1) for using a tertiary amine polyurethane resin as a lower magnetic layer or a method (see Patent Document 2) for using a polyacetal resin as an upper magnetic layer or the like is disclosed. Many investigations of the methods have been hitherto carried out. Further, a method (see Patent Document 3) for improving a durability and traveling characteristics by similarly using as an additive agent carbon black having average particle size located within a specific range is disclosed.
However, in the above-described VTR or the like for the broadcasting station, heads as many as 20 to 50 are mounted on a rotary drum to provide a compatibility for both formats of a conventional analog system and a digital system. Further, there exist VTRs having relative speeds different from those of heads depending on an NTSC system and a PAL system. Therefore, a request for the durability of a magnetic tape increasingly becomes severe.